The current electrophotography in which an electrostatic latent image formed on an image-carrying member is visualized by development in order to obtain a final image can be generally categorized into two developing processes; a method using a one-component developer and a method using a two-component developer comprising magnetic carrier and non-magnetic toner.
Among one-component developer developing processes, especially in a developing process employing one-component, nonmagnetic developer, the force holding developer on a development sleeve solely comprises the Coulomb force and the van der waals force. Therefore, it is difficult to constantly form a uniform developer layer. Furthermore, with a developing process with one-component magnetic developer, it is difficult to obtain a brilliant color toner other than of black, since the toner usually contains magnetic substance which is black colored magnetite powder. In contrast, when compared to other developing processes, a developing process using a two-component developer can constantly form a stable developer layer and allow the use of brilliant color toner.
The developing process using a two-component developer is further categorized into two methods, the contact developing process and the non-contact developing process; in the former process, an electrostatic latent image on an image-carrying member is directly contacted with a developer layer which is in the form of turf (magnetic brush) and formed on a developer carrying member (development sleeve), thereby the toner powder adhering on magnetic carrier particles is allowed to leave the carrier and deposit on the latent image; in the latter process, an image-carrying member and a developer layer are kept in a non-contact basis, thereby a high-frequency developing bias having a direct current component is exerted to between the image-carrying member and the developer layer in order perform developing by allowing a toner to jump.
In the non-contact developing process using a two-component developer, the developing is performed without allowing a developer layer to come in contact with an imagecarrying member. Therefore, this developing process is advantageous especially when forming a multi-color image, as a plurality of developing units can be disposed along the circumferential surface of the image-carrying member, thereby the corresponding number of developing steps are performed in order to form the same number of toner images on the same position of the image-carrying member.
When developing an electrostatic latent image on an image-carrying member with developer held on and transported by a development sleeve, the smaller gap between the image-carrying member and the development sleeve can enhance the contrast in obtained image, improving the image quality. In a contact developing process in which the developing is performed by allowing the developer layer to contact with the image-carrying member, the maller gap between image-carrying member and the development sleeve, however, it is necessary to limit the amount of developer being held on and transported by the development sleeve to an appropriate smaller range, in order to prevent the developer from being compacted between the image-carrying member and the development sleeve or an electrostatic latent image or developed image from being damaged by the pressure or rubbing force of developer. The term "amount of developer" means a per unit area weight of developer held on the development sleeve.
Also, in the non-contact developing process in which the developing is performed by constantly separating the image-carrying member from the developer layer, it is necessary to limit the amount of developer being held on and transported by the development sleeve to an appropriate smaller range, in order to prevent the image-carrying member from coming into contact with the developer layer.
A conventional developing process using a two-component developer performs the developer with a developing unit having, for example, the following constitution. In the similar developing unit, a development sleeve is made of a nonmagnetic cylindrical member or a non-magnetic belt, whereby to the side opposite to the image-carrying member on the development sleeve is disposed a magnet. This magnet may be either a stationary magnet or a magnet rotatable on a specific axis, according to a specific development requirement. Among the developer held on the development sleeve, the magnetic carrier receives a magnetic force derived from the magnetic field of the magnet which is in a position on the development sleeve opposite to the image-carrying member, and forms a turf, thereby the carrier is held on the development sleeve. Being agitated in a developer, the non-magnetic toner is rubbed with the magnetic carrier and acquires triboelectrical charge. This means the non-magnetic toner acquires triboelectricity of which polarity opposite to the magnetic carrier, thereby the non-magnetic toner and the magnetic carrier are bonded together by a Coulomb force. Since the magnetic carrier is held on the development sleeve by the magnetic field of the magnet, the non-magnetic toner together with the magnetic carrier is held on the development sleeve, thus the development sleeve is provided with a developer layer comprising both the magnetic carrier and the non-magnetic toner. The developer layer is held on and carried by the development sleeve and develops an electrostatic latent image formed on the iamge carrying member.
Conventionally, an ordinary development sleeve is composed of a cylinder with a diameter of approximately 50 mm and has less than eight magnetic poles. However, for example, in a color copying apparatus in which a plurality of color toner images are formed in a same position, a plurality of developing units disposed along the circumferential surface of a photosensitive drum requires a miniaturization of individual developing unit. Accordingly, a developing unit with diameter of 15 to 30 mm is advantageously used. A small diameter development sleeve naturally has a shorter circumferential length, and accordingly has a smaller number of magnetic poles; a most commonly used small diameter development sleeve comprises a rotatable magnetic roller having, in total, alternately arranged 4 to 6 N and S poles.
By such a magnetic roller, a developer layer adhering to the circumferential surface of development sleeve is transported to a development region which is facing an image-carrying member. The developer layer should satisfy the minimum criteria mentioned previously; the developer is not compacted in the gap between the image-carrying member and the development sleeves; and, the image-carrying member and the developer layer are strictly separated with each other. Furthermore, the developer layer is required to satisfy the following criteria. (1) The "fogging", which is the phenomenon where toner adheres to the surface of image-carrying member othre than in the image region, does not occur. (2) The "carrier adhesion", which is the phenomenon where carrire adheres to the surface of image-carrying member, does not occur. (3) A sufficient amount of toner being capable of providing satisfactory image density deposits on an electrostatic latent image region on the image-carrying member. (4) The toner image made of deposited toner has tones which satisfactorily reproduce the tones of electrostatic latent image.
To satisfy these criteria, the following conditions should respectively satisfy a specific requirement: the physical properties of developer and a developer agitation method; the physical properties of image-carrying member and the relative velocity between the image-carrying member and the developer layer; the means for forming an electrostatic latent image on the image-carrying member; the electric bias exerted between the image-carrying member and the development sleeve; the magnetic force of magnet and the pattern of magnetic field. The requirements for satisfying the above criteria include the amount of developer on the development sleeve, the thickness of developer layer; the permissible scope of these two requirements is stricter when compared to the previously mentioned minimum criteria.
Conventionally, to achieve a required amount and thickness of developer in the developer layer held on the development sleeve, a developer layer-thickness regulating member 205 shown in FIG. 8 made of metal of resin is disposed in the close vicinity of the development sleeve 203 surface. The developer D held and travelling on the surface of development sleeve 203 by way of magnetic force passes through the gap between the developer layer-thickness regulating plate 205 and the development sleeve 203 surface, thereby the thickness of developer layer is regulated and the layer is transported to the development region.
The development sleeve 203 here is composed of a nonmagnetic cylinder having a built-in magnetic roller 204.
With a system using such a developer layer-thickness regulating plate 205, when keeping the thickness of developer D approximately at 1 mm or smaller, the gap between the developer layer-thickness regulating plate 205 and the development sleeve must be kept approximately 0.5 mm or smaller. This is because the developer D acquires a high density, when passing between the developer layer-thickness regulating plate 205 and the development sleeve 203, due to pressure with which the developer is transported, and, in contrast, after passing between the developer layer-thickness regulating plate 205 and the development sleeve 203, the developer D on the development sleeve 203 acquires a low density and rises due to the magnetic force of magnetic roller 204, thereby the thickness of developer layer formed with the same amount of developer becomes larger.
An ordinary copying apparatus or a printer, with which an electrostatic latent image is visualized by the developing process, has an image width of several hundred millimeters. Such an apparatus has a disadvantage; constantly keeping the gap between the developer layer-thickness regulating plate and the development sleeve uniformly at approximately 0.5 mm or smaller along the whole image width requires high-precision machining for the layer-thickness regulating plate and high-precision adjustment thereof, and this is not easily performed.
Additionally, the non-contact developing system using a two-component developer has a specific disadvantage; since the height of carrier turf in the development region is larger than that of the non-contact developing system using a one-component developer, it is impossible to use a smaller development gas, hence the poor reproducibility of fine lines or fine characters.
Conventionally, the non-contact developing process has poor reproducibility of fine lines or fine characters, due to an electrical field in the developing area. This is because, in a non-contact developing process, the edge electrical field generated on the edge region of an electrostatic latent image formed on an image-carrying member acts as a barrier against toner, while in a contact developing process such an electrical field attracts more toner powder. This advantage, however, is resolved by narrowing the development gap, which is the gap between the development electrode and the image-carrying member. However, in the developing process using a two-component developer, the height of carrier fur is, as mentioned previously, larger than that of a developing process using a one-component magnetic toner, and, resultingly, such a countermeasure has a limitation, as the narrower development gap means the carrier fur coming into contact with the image-carrying member.
To solve the above problems, the applicant proposed in Japanese patent Applications Nos. 34318/1986 and 31319/1986 and the like a technical means, appropriate for the noncontact developing system, for forming a developer layer on a development sleeve.
To be brief, according to the technical means, the tip of resilient plate attached to a supportive member is positioned toward the upstream side, relative to the travel of developer on a development sleeve, thereby the resilient plate is pressed onto the development sleeve in order to regulate the thickness of travelling developer composed of magnetic carrier and toner. This technique regulates the developer layer thickness to a thinner level with high-precision when compared to other, conventional developer layer-thickness regulating means.
According to the above arrangement, the developer thickness on the development sleeve is regulated with high precision when compared to a conventional developer layer-thickness regulating means. In the case of developing process using a two-component developer, however, the height of magnetic carrier turf is often larger than that of one-component magnetic toner. In addition to the occurrence of fogging by toner which was mentioned previously, the developing system using a two-component developer may also cause the problem of carrier adhesion. Therefore, when an image-carrying member is in the close vicinity of the development sleeve in order to allow the electrical field to exerts its force more strongly on the toner and carrier which respectively have a polarity reverse to that of their counterpart, and resultingly, to suppress edge-effect and to reproduce a solid black area or characters more definitely, another problem arises; the developer comes into contact with the surface of image-carrying member, and, especially with a multi-color copying system, readily causes a color mixed with another color as well as the carrier adhesion, thus deteriorated image quality.